STIS currently supports four basic operating modes:
ACCUM operating modes for the CCD and MAMAs, which produce a time- integrated accumulated image. These are the most commonly used modes.
TIME-TAG operating mode for the MAMA detectors, which outputs an event stream of high-time-resolution observations in the UV.
ACQ (acquisition) and ACQ/PEAKUP operating modes for the CCD and MAMAs used to acquire targets in the spectroscopic slits and behind coronographic bars and masks. Target acquisitions are described further in Chapter 8.
The STIS CCD has only the single operating mode, ACCUM, for images and spectroscopy. The CCD pixels accumulate charge during the exposure in response to photons. The charge is read out at the end of the exposure and converted to 16 bit data numbers (DN) at a selectable gain (number of electrons per DN) by the A-to-D converter. The DN are stored as 16 bit words (with a range 0 to 65,535) in the STIS data-buffer memory array. At the default CCDGAIN=1, the gain amplifier saturation level (33,000 e-), and not the 16-bit format, limits the total counts that can be sustained in a single exposure without saturating (see also Analog-to-Digital Conversion: Selecting the CCDGAIN and CCD Saturation). At the other supported gain, CCDGAIN=4, the CCD full well (144,000 e- or 36,000 DN, except in the outermost regions of the CCD where the full well is 120,000 e-, see Chapter 7 for details) still determines the saturation limit.
A full detector readout is actually 1062 x 1044 pixels with physical and virtual overscans. Scientific data are obtained on 1024 x 1024 pixels, each projecting to ~0.05 x 0.05 arcsecond on the sky. The dispersion axis runs along axis1 (image x or along a row of the CCD), and the spatial dimension of the slit runs along axis2 (image y or along a column of the CCD). Figure 11.1 illustrates the full CCD format and its orientation with respect to the spacecraft (U2 and U3 or V2 and V3) axes. Arrows indicate the orientation of the parallel and serial clocking. The readout directions depend on the amplifier used. For the default amplifier D, the readout is at the upper right corner. It includes 19 columns of leading and 19 columns of trailing physical overscan in axis1, and 20 trailing rows of virtual overscan in axis2. The trailing serial overscan as well as the parallel overscan pixels are used to determine the bias level in post-observation data processing. The parallel overscan can also be used in the diagnosis of charge-transfer problems.
The minimum CCD exposure time is 0.1 second and the maximum possible exposure time is 4.7 hours (though we cannot imagine wanting a single exposure longer than 60 minutes). The minimum time between identical exposures for CCD full-frame (1062 x 1044) images is ~45 seconds.1 This time is dominated by the time it takes to read out the CCD (29 seconds for the full frame) and can be reduced to ~19 seconds if you use a subarray (see CCD Subarrays).
The CCD supports on-chip binning. When on-chip binning is used the specified number of pixels in the serial and parallel directions is read out as a single pixel. The advantage of CCD binning is that the read noise per binned pixel is the same as the read noise per unbinned pixel. Thus if your signal-to-noise per pixel is dominated by read noise when no binning is used, you can increase the signal-to-noise by binning. The disadvantages of using on-chip binning are (a) that it reduces the resolution of your spectrogram or image, (b) that the relative number of pixels affected by cosmic rays increases, and (c) that the relative number of `hot' pixels (which is ~1% of all CCD pixels for unbinned data by the time this handbook is issued, see Chapter 7), increases by a factor proportional to the binning factor. On-chip binning of 1, 2, or 4 pixels in both the AXIS1 and AXIS2 directions is supported. Note that on-chip binning is not allowed when subarrays are used.
The number of hot pixels has been increasing steadily with time due to accumulated radiation damage on the STIS CCD (see the discussion on hot pixels in Chapter 7). Thus the impact of hot pixels on binned data has become significantly larger. Also note that when spectral data are spatially rectified, a single pixel in the original data will be interpolated into four pixels in the rectified image. For data binned NxM on board the spacecraft, a single bad pixel will, after rectification, affect the equivalent of 4xNxM pixels in an unbinned image.
When using the ETC to estimate the effects of on-board binning on the S/N of CCD observations, be aware that increasing the binning in the dispersion direction may cause the ETC to use a larger resolution element for its S/N calculation. Be sure to understand how much of any increase in the S/N number output by the ETC is due to an actual decrease in the read noise and how much is simply due to a change in the size of the resolution element assumed for the calculation.
During Phase II, you specify the binning for your CCD observations using the BINAXIS1 and BINAXIS2 optional parameters. The default values are 1.
Subarrays can be used when the CCD detector is read out. Generally, there is no need to use a subarray for STIS data. The main scientific use of CCD subarrays is for time-resolved optical spectroscopy, where subarrays can be used to reduce the CCD read time and keep the data volume at a manageable level. Another reason to use CCD subarrays is to limit the effect of imperfect Charge Transfer Efficiency (see Charge Transfer Efficiency). CCD subarrays can also be specified for CCD acq/peak observations to limit the region in a diffuse object (e.g., a galaxy) over which the flux is summed for the peakup. When a subarray is used, only the portion of the detector which is within the specified subarray is read out and transmitted to the ground (see Figure 11.2-note that the spectrogram curvature is exaggerated in this figure).
As described in CCD ACCUM Mode, full-frame CCD readouts are composed of 1062 x 1044 pixels: 1024 x 1024 data pixels, 19 leading and 19 trailing serial-overscan pixels, and 20 trailing parallel-overscan pixels. Dispersion runs along axis1 and the long dimension of the slit runs along axis2. Subarrays are required to span the full width of the CCD detector in the serial (dispersion) direction in order to ensure they contain the serial overscan needed to determine the bias level; however, you can control the height of the subarray in the parallel direction (i.e., along the slit for long-slit spectroscopic observations). Note that no parallel overscan is returned for subarrays (see also CCD Bias Subtraction and Amplifier Non-Linearity). Subarray sizes and centers, as specified in Phase II, are given by these parameters:
sizeAXIS2 - size in pixels of the subarray in the axis2 direction.
centerAXIS2 - central pixel of the subarray in the axis2 direction.
The minimum allowed value of sizeAXIS2 for ACCUM mode observations is 30 pixels (corresponding to 1.6 arcsec), and sizeAXIS2 must be an even number of pixels. The subarray is centered on the target position.
Use of Subarrays to Reduce the CCD Read Time
The minimum time between identical CCD exposures is the readtime + 16 seconds. The time to read out a CCD subarray is:
Thus, using the smallest available subarray, which is 32 pixels high, you can reduce the minimum time between identical exposures to ~19 seconds (16 seconds overhead plus 3 seconds read time). The minimum time between full-frame CCD exposures is 16 + 29 = 45 seconds.
Use of Subarrays to Reduce Data Volume
The format of the data you receive when you use a CCD subarray will have dimensions 1062 x sizeAXIS2, will cover the full range in the dispersion direction, and will include the serial overscan. The STIS buffer can hold eight full-frame CCD exposures at one time, or 8 × (1024 / sizeAXIS2) exposures at any one time. Full-frame CCD data acquired in one exposure can be transferred to the HST data recorder during the subsequent exposure(s) so long as the integration time of the subsequent exposure is longer than 3.0 minutes. If you are taking a series of exposures which are shorter than that, the buffer cannot be emptied during exposure, and once the STIS buffer fills up, there will be a pause in the exposures sequence of roughly 3 minutes as the buffer is emptied. This problem can sometimes be avoided with the judicious use of subarrays.
In MAMA ACCUM mode exposures, photons are accumulated into a 2048 x 2048, 16 bit per element over-sampled array in the STIS data buffer memory as they are received. At the end of the exposure, the data can be left in the over-sampled (or highres) format, which is the default for scientific exposures, or they can be binned along axis1 and axis2 to produce a 1024 x 1024 native-format image. ACCUM is the mode of choice for all observations that do not require time resolution on minute or less scales. Dispersion runs along AXIS1 and the spatial dimension of the slit (and the orders for echelle observations) run along axis2. Figure 11.3 and Figure 11.4 illustrate the format and coordinate system for MAMA images, showing how first-order and echelle ACCUM mode spectrograms appear. prism images have dispersion along axis1. Note that for FUV-MAMA G140L and G140M the spectrograms are shifted low-resolution 120 pixels down in AXIS2 to AXIS2=392 to ensure that they will not fall on the shadow of the repeller wire (see MAMA Spectral Offsetting). Thus there will be ~3 arcseconds less spatial sky coverage to decreasing AXIS2 and ~3 arcseconds more spatial sky coverage to increasing AXIS2 along the slit. Said another way, slit center will project ~3 arcseconds below the detector center along AXIS2 for G140L and G140M observations. Note also the effects of the monthly offsetting which applies to all MAMA modes (see MAMA Spectral Offsetting). (Data taken prior to March 15, 1999 have the slit center offset above the detector center.)
The minimum MAMA ACCUM mode exposure time is 0.1 second and the maximum exposure time is 1.8 hours. The minimum time between identical MAMA ACCUM exposures is ~30 seconds, for exposures which are longer than 3 minutes, and it is 2.5 minutes for exposures which are shorter than 3 minutes. This difference arises because in the former case the buffer can be dumped to the HST recorder during the subsequent exposure (i.e., in parallel), but in the latter case there is insufficient time to dump the buffer during the subsequent exposure and the buffer must be dumped serially (i.e, using observing time).
For the MAMA medium-resolution, first-order modes and medium and high-resolution echelle modes (i.e., gratings G140M, G230M, E230M, E230H, E140M, and E140H), a correction for Doppler shifting of the photon energies due to HST spacecraft motion is applied as the photons are counted, prior to their addressing in STIS data-buffer memory. The leading and trailing pixels in the dispersion direction (axis1) for Doppler-corrected exposures therefore receive less effective integration time, since source photons at the corresponding wavelengths have been Doppler-shifted off the edge of the detector for some fraction of the total exposure time. This effect is strongest in the high-resolution echelle modes, where for a maximum HST spacecraft velocity of 7.5 km sec-1, the leading and trailing ~20 axis1 pixels will have reduced effective exposure times.
The MAMA detectors record scientific data in the so-called highres mode, producing 2048 x 2048 images of super resolution - one half the 1024 x1024 "native"-format pixel size defined by the anode readout itself. All scientific data are taken in this format by default. Below we explain in more detail the nature of highres data.
The MAMA detectors have 1024 x 1024 physical or so-called native-format pixels. However, each count is detected by multiple electrodes, so the charge distribution among the electrodes can be used to centroid the incident charge cloud to subpixel resolution. The gain of the highres 2048 x 2048 mode is a ~10-30% increase in resolution at the price of decreased signal-to-noise ratio per pixel arising from the increased fixed-pattern noise of the statistics of charge partition among the electrodes. The highres flat fields have much more structure than the 1024 x 1024 flats, with adjacent columns and rows differing by ~30% in an off/on pattern whose time variability is appreciably higher than for 1024 x 1024 format images. This effect and the inherently lower signal-to-noise ratio in the full-resolution flat-field images (nominally ~20 to 1 per highres pixel) suggest that it may be difficult to routinely realize the benefit in resolution. Highres is most likely to have application at low to intermediate signal-to-noise ratios (below 20:1), low to intermediate incident count rates (exact range to be determined - expected to be applicable only at rates less than 5-10 counts sec-1 pixel-1) and in programs which need the highest resolving power in spectroscopy. However, we note that data taken in highres mode can always be binned to 1024 x 1024 on the ground in post-observation data processing, and since the extra overheads in highres mode are typically quite small, highres is the default data-taking mode for the MAMA. The pipeline bins the data to 1024 x 1024 format during calibration so that the pipeline-output calibrated images are native format (see the HST Data Handbook for more details).
TIME-TAG mode is used for high-time-resolution spectroscopy and imaging in the ultraviolet. When used in TIME-TAG mode, the MAMA produces an event stream of axis1, axis2, and time data points, with a time resolution of 125 microseconds. The volume of data produced in TIME-TAG mode can be very large and the data therefore must be continuously transferred from the STIS internal buffer to the data recorders to sustain TIME-TAG exposures of any significant duration.
The axis orientation in TIME-TAG is the same as in ACCUM mode (see page 217). The spacecraft time (absolute zero point of the time) is routinely known to 10 millisecond accuracy. No Doppler correction is applied by the flight software for TIME-TAG mode, but the correction can be applied during the post-processing of the data. The recorded times are the spacecraft times, which can be converted to heliocentric times using the ephemeris of the Earth and the spacecraft. TIME-TAG mode is illustrated in Figure 11.5. Processing of TIME-TAG data by the STScI pipeline is described in Pipeline Processing Overview.
There are several limitations in TIME-TAG mode of which users should be aware:
TIMETAG exposures of any substantial duration (>3 minutes). This restriction does not apply to count rates below and equal to 21,000 count sec-1.
TIME-TAG exposure is 4 x 106 / R seconds, where R is the count rate from the source. The MAMA detectors are not able to count reliably at global count rates greater than 30,000 count sec-1 in TIME-TAG mode (i.e., uncorrectable non-linearity and mis-time-tagging sets in).
TIME-TAG mode is 6.0 x 107 / R seconds. For example, at count rates of R = 21,000 count sec-1 the maximum exposure time in any one visit is ~48 minutes. In special cases, for special scientific needs, the HST data recorder can be devoted exclusively to a single observing program for a predetermined period of time, allowing longer total exposure durations. Requests for such special handling should be included in your Phase I proposal justification.
The ground system must know the data rate to expect in order to schedule data transfers. Transfers are done in blocks of 8 megabytes (half the buffer capacity) which corresponds to 2 x 106 TIME-TAG events. The frequency of scheduled dumps depends on observer-specified information about the expected count rate. Specifically, during Phase II, TIME-TAG observers must specify a BUFFER-TIME parameter, where BUFFER-TIME is the minimum time during which half the STIS buffer may be filled, i.e., the minimum time to accumulate 2 x 106 TIME-TAG events. Thus, to a first approximation:
where the parameter R, to a first approximation, is the maximum expected event rate in counts sec-1 from all the sources in the image and any sky plus detector backgrounds. ( Figure 11.6 provides a flow-chart for a more exact estimation of the BUFFER-TIME.)
Explicitly, for calculation of BUFFER-TIME (Phase II), R should be determined as the maximum mean countrate in any BUFFER-TIME interval. Thus, R is the maximum mean count rate sustained over any BUFFER-TIME interval during your exposure, which can be approximated as the maximum instantaneous count rate, so long as the source does not vary dramatically.
If your estimated value of R is smaller than the actual maximum mean count rate, your estimated BUFFER-TIME will be larger. As a result, you will fill the STIS internal memory faster than it can be emptied to the HST data recorder. This will inevitably lead to some loss of data, and there is some uncertainty at present on the exact amount of data lost in such a case. You should also note that, at the end of each BUFFER-TIME, any unfilled part of the allocated STIS buffer (8 megabytes, equivalent to 2 x 106 counts) is filled with fill-data (zeros) before the buffer memory is dumped to the HST data recorder. If your estimated value of R is larger than the actual value, your estimated BUFFER-TIME will be smaller, and each buffer will contain some fill data which will later be discarded by the flight software. This is harmless in the sense that this will not lead to any data loss. However, your total exposure time will be restricted unnecessarily since the total integration time per visit should not be larger than 30 times the BUFFER-TIME. (As explained above, for special scientific needs, longer total exposure durations can be permitted but requests for such special handling should be included in your Phase I proposal justification.)
In summary, your BUFFER-TIME estimate should satisfy the following conditions:
BUFFER-TIME > 99 seconds for continuous TIME-TAG observations longer than ~3 minutes.
BUFFER-TIME < 2 x 106 / R sec (so that all the counts can be recorded).
BUFFER-TIME (Under special circumstances this condition can be waived as explained in the text, if approved during Phase I).
BUFFER-TIME < 0.5 x the total integration time (if your calculated BUFFER-TIME comes out to be larger than half the total integration time, just use half the integration time as your BUFFER-TIME. There is no reason to make the BUFFER-TIME any larger.)
The flowchart given in the next subsection takes all these effects into account in estimating the BUFFER-TIME.
First, calculate the value of R, the maximum expected countrate from all the sources in the image. Make sure you take the contribution from the dark current and sky-background (including the geocoronal line emissions) into account in calculating R. As an extra safety margin, increase R by about 20% if possible, which will ensure that all the events are recorded. Then follow the flowchart in Figure 11.6 to estimate the BUFFER-TIME.
CR-SPLIT, i.e., if CR-SPLIT=3, there will be 3 x 45 seconds of overhead on the set of 3 exposures due to CCD setup and readout.
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